THE PURPOSE FOR THESE EXPERIMENTS IS TO DETERMINE THE MECHANISM OF MAGNESIUM TRANSPORT IN THE RENAL PROXIMAL TUBULE CELLS OF THE FUNDULUS HETEROCLITUS KIDNEY. THE PROXIMAL TUBULE CELLS ARE ISOLATED USING A DIVALENT FREE RINGER SOLUTION AND AN ENZYME DIGESTION METHOD. THE MAGNESIUM CONCENTRA-TIONS OF MARINE TELEOST URINE IS AS HIGH AS 150MM. EXTRACELLULAR MAGNESIUM CONCENTRATIONS IN TELEOST FISH ARE VERY TIGHTLY CONTROLLED, AND SEEM TO EXCLUSIVELY REGULATED BY THE KIDNEY. IT IS KNOWN THAT THERE IS AN ACTIVE TRANSPORT MECHANISM FOR THE TRANSEPITHELIAL TUBULAR SECRETION OF MAGNESIUM, AND ISOLATED PERFUSED PROXIMAL TUBULES SHOW AN INVERSE RELATIONSHIP BETWEEN THE CONCENTRATIONS OF SODIUM AND MAGNESIUM IN THE TUBULES LUMEN, WHICH IS CONSISTENT WITH AN EXCHANGE MECHANISM. HOWEVER, SINCE THE RENAL PROXIMAL TUBULES ARE HIGHLY PERMEABLE TO SODIUM VIA PARACELLULAR PATHWAYS, THE SODIUM/MAGNESIUM EXCHANGE MECHANISM IS NOT SUFFICIENTLY SUPPORTED BY THE CURRENTLY AVAILABLE DATA. IN ORDER TO ELIMINATE THE CONFOUNDING INFLUENCES OF PARACELLULAR FLUX, WE ARE MEASURING THE MAGNESIUM SECRETIONS FROM ISOLATED PROXIMAL TUBULE CELLS USING MAGNESIUM ION SELECTIVE VIBRATING LIQUID ION-EXCHANGE ELECTRODES. IF THE MAGNESIUM TRANSPORTER IS AN ATP DEPENDENT MECHANISM, THEN THE POSSIBILITY EXIST OF IT BEING EITHER PRIMARILY (A PUMP) OR SECONDARILY (AN EXCHANGER) DEPENDENT ON THE ATP HYDROLYSIS. IN ORDER TO DETERMINE THE TRANSPORT MECHANISMS, WE PROPOSE THE FOLLOWING MANIPULATIONS TO OBSERVE THE AFFECTS ON THE MAGNESIUM SECRETION. WE ARE GOING TO MODIFY THE METABOLIC STATE OF THE CELL BY STIMULATING OR INHIBITING THE PRODUCTION OF ATP. IN ORDER TO DETERMINE THE DEPENDENCE OF THE MECHANISM ON SODIUM, WE ARE GOING TO MANIPULATE THE EXTRACELLULAR SODIUM CONCENTRATIONS TO TEST FOR POSSIBLE COUPLING BETWEEN SODIUM AND MAGNESIUM. WE ARE ALSO GOING TO TEST THE INFLUENCES THE ATP AND SODIUM CONCENTRATIONS HAVE IN CONJUNCTION WITH EACH OTHER. FURTHERMORE, WE ARE ATTEMPTING TO DETERMINE IF THE ISOLATED CELLS ARE CAPABLE OF RESPONDING DIRECTLY TO ELEVATED BATH MAGNESIUM CONCENTRATION OR IF THE INCREASED MAGNESIUM SECRETION IS MEDIATED BY HORMONES ON NEUROTRANSMITTERS.